Reduced yellowing electrodepositable coating composition

ABSTRACT

Electrodepositable coating compositions comprising an active hydrogen-containing ionic resin and a curing agent which contains blocked isocyanate groups and oxazolidone groups are disclosed. These coating compositions reduce the tendency of cured electrodeposited films to cause yellowing in subsequently applied topcoat systems. The use of these electrodepositable coating compositions in the process of electrodeposition is also disclosed.

This is a division of application Ser. No. 07/962,568, filed Oct. 19,1992 U.S. Pat. No. 5,212,189.

FIELD OF THE INVENTION

The present invention relates to electrodepositable coating compositionsand to their use in the process of electrodeposition. More specifically,the present invention relates to electrodepositable coating compositionswhich contain curing agents that provide resistance to yellowing whenthe composition is electrodeposited and cured.

BACKGROUND OF THE INVENTION

Certain aqueous coating compositions can be applied by the process ofelectrodeposition. Electrodeposition is the process whereby a coatingcomposition is placed in contact with an electrically conductive anodeand an electrically conductive cathode with the surface to be coatedbeing either the anode or the cathode. Following contact with thecoating composition, an adherent electrically insulating film isdeposited on one of the electrodes when a sufficient voltage isimpressed between the electrodes. Electrodepositable coatingcompositions can be applied to a variety of electroconductive substratesespecially metals such as steel, aluminum, copper, magnesium) andconductive carbon-coated materials.

Electrodepositable coating compositions which are applied cathodically(that is, the surface to be coated is the cathode) are preferred tothose which are applied anodically because cationic compositions usuallyprovide superior corrosion resistance. Electrodepositable coatingcompositions based on epoxy-derived resins which are crosslinked withmaterials containing blocked isocyanate groups upon application of heatare very cox,]on in the automotive industry.

Often, a topcoat system is subsequently applied to electrodeposited,cured coatings in order to achieve even greater corrosion resistance anddurability than the electrodeposited coating alone would provide.Additionally, topcoat systems are applied in order to provide anattractive, glossy finish which must match a specific color standard.This is especially true in the automotive industry, where, for example,a non-conductive plastic part which has not been electrocoated abuts ametal part which has been electrocoated, and the color of these partsmust match.

Thermosetting topcoat systems which contain acrylic, polyester and/oralkyd resins that crosslink with aminoplast or blocked polyisocyanatecuring agents are very common in the automotive industry. These topcoatscan be spray-applied as a single color coat or as a two-coat base/clearsystem. It is well known in the art that certain light-colored topcoatsystems such as whites, light blues, creams or tans are subject toyellowing when they are applied over certain cured electrodepositablecoating compositions. Such yellowing can be caused by various factorssuch as the electrodeposition coating composition, the topcoatcomposition or the chemical composition of the atmosphere in which thecoating compositions are cured, Yellowing which is related to theelectrodeposition coating composition will hereinafter be referred to as"ED-related yellowing."

ED-related yellowing is especially pronounced when low molecular weightaromatic blocked isocyanate curing agents are used in theelectrodepositable coating composition. Aliphatic blocked isocyanatecuring agents produce electrodepositable coating compositions whichcause less ED-related yellowing in the subsequently applied topcoatsystems, but these curing agents are generally more expensive thancertain aromatic blocked isocyanate curing agents such as toluenediisocyanate (TDI). Aromatic blocked. isocyanate curing agents whichhave higher molecular weight than TDI also produce electrodepositablecoating compositions which tend to cause less ED-related yellowing inthe subsequently applied topcoat systems.

The present invention is based on the discovery that blocked isocyanatecuring agents which contain oxazolidone groups in addition to blockedisocyanate groups produce very little ED-related yellowing insubsequently applied topcoat systems. This allows formulation ofnon-yellowing or reduced yellowing electrodepositable coatingcompositions with a wide variety of blocked isocyanate curing agents,particularly blocked aromatic isocyanate curing agents based onrelatively low molecular weight aromatic isocyanates such as TDI, whichare non-yellowing.

SUMMARY OF THE INVENTION

The present invention is directed toward-an electrodepositable coatingcomposition which minimizes ED-related yellowing in subsequently appliedtopcoats. The invention also describes a method for coating a conductivesubstrate by the process of electrodeposition using theelectrodepositable coating compositions described herein in order toproduce an electrocoated article.

The electrodepositable coating composition comprises:

A. an active hydrogen-containing ionic resin; and

B. a curing agent which contains blocked isocyanate groups and groups ofthe following structure: ##STR1##

As used herein, the term "ED-related yellowing" generally refers to thetendency of certain electrodepositable coating compositions to cause ayellowed color in a subsequently applied topcoat system during thecuring process of said topcoat system-ED-related yellowing is of twotypes. One type is so-called "fume-yellowing", in which it is believedthat a volatile species derived from an isocyanate moiety condenses inintricate areas on the automobile body upon curing of theelectrodeposited coating. When the topcoat is subsequently applied andcured the isocyanate-derived product again volatilizes and fouls thetopcoat. Another type of ED-related yellowing is so-called"bleed-through yellowing", in which it is believed that decompositionproducts of the blocked isocyanate curing agents penetrate into thetopcoat through an intermediate sealant layer.

DETAILED DESCRIPTION

The active hydrogen-containing resins of the present electrodepositablecomposition include both anionic resins and cationic resins with thecationic resins being preferred because they usually provide superiorcorrosion resistance. The ionic resins should contain active hydrogenssuch as those provided by hydroxyl, primary amino secondary amino andthiol groups, including mixtures thereof. The active hydrogens arereactive with blocked isocyanate groups resulting in a curing reactionwhen the coating compositions are heated. Particularly preferred activehydrogen group containing ionic resins are cationic resins which containamine salt groups such as the acid-solubilized reaction products ofpolyepoxides and primary or secondary amines as described in U.S. Pat.Nos. 3,663,389; 3,922,253; 3,984,299; 3,947,388; 3,947,339 and4,031,050.

Besides the epoxy-amine reaction products, active hydrogen-containingionic resins can also be selected from cationic acrylic resins such asthose described in U.S. Pat. Nos. 3,455,806 and 3,928,157. Furtheractive hydrogen-containing ionic resins can be selected from cationicpolyester resins, and the like, which contain ionic groups and activehydrogen groups.

Besides resins containing amine salt groups, resins containingquaternary ammonium salt groups can also be employed. Examples of theseresins are those which are formed from reacting an organic polyepoxidewith a tertiary amine salt. Such resins are described in U.S. Pat. Nos.3,962,165; 3,975,346; 4,001,101 and 4,101,486. Examples of othercationic resins are ternary sulfonium salt group-containing resins andquaternary phosphonium salt group-containing resins such as thosedescribed in U.S. Pat. Nos. 3,793,278 and 3,984,922, respectively.

The resins to which the present invention is particularly effective arethose cationic resins which contain primary and/or secondary aminegroups. Such resins are described in U.S. Pat. Nos. 3,663,389; 3,947,339and 4,116,900. In U.S. Pat. No. 3,947,339, a polyketimine derivative ofa polyamine such as diethylenetriamine or triethylenetetraamine isreacted with a polyepoxide. When the reaction product is neutralizedwith acid and dispersed in water, free primary amine groups aregenerated. Also, equivalent products are formed when polyepoxide isreacted with excess polyamines such as diethylenetriamine andtriethylenetetraamine and the excess polyamine vacuum stripped from thereaction mixture. Such products are described in U.S. Pat. Nos.3,663,389 and 4,116,900.

Modified resins such as those obtained by chain extending thepolyepoxide to increase its molecular weight are also preferred in thepractice of the invention. Such materials are described in U.S. Pat. No.4,148,772 in which the polyepoxide is chain extended with a polyesterpolyol and in U.S. Pat. No. 4,468,307 in which the polyepoxide is chainextended with a particular polyether polyol. Also, chain extensionmethods such as those disclosed in Canadian Patent 1,179,443 can beused.

The epoxy polymers which are used in preparing the cationic resins arepolyepoxides, that is, polymers having an epoxy equivalency greater than1, preferably about 2 or more. Preferred are polyepoxides which contain1,2-epoxide groups and which are difunctional with regard to epoxy. Thepreferred polyepoxides are polyglycidyl ethers of cyclic polyols.Particularly preferred are polyglycidyl ethers of polyphenols such asbisphenol A.

Besides the polyglycidyl ethers of polyphenols, epoxy-containingpolymers which can be used are acrylic polymers which contain epoxygroups. These polymers are formed by polymerizing an unsaturated epoxygroup-containing monomer such as glycidyl acrylate or glycidylmethacrylate with one or more polymerizable ethylenically unsaturatedmonomers. Examples of these polymers are described in U.S. Pat. No.4,001,156.

The oxazolidone group-containing curing agents contain groups of thefollowing structure: ##STR2##

More specifically, the curing agents contain groups of the followingstructure: ##STR3## where X is the residue of an isocyanate, preferablya polyisocyanate, more preferably an aromatic isocyanate, mostpreferably an aromatic polyisocyanate. Preferably, the curing agentcontains groups of the following structure: ##STR4## where X is asdescribed above; and where A is a (are) blocked isocyanate group(s),preferably of the structure: ##STR5## where D is a radical derived fromthe group consisting of alcohols, including glycol ethers and aminoalcohols, lactams, ketoximes, aliphatic amines and beta-dicarbonylcompounds, and where Y and Y' are different and are hydrogen and aresidue derived from an epoxy resin, preferably a polyepoxide.

The X groups in the above-mentioned structural formulas are derived fromisocyanates which can be aliphatic or aromatic isocyanates, preferablypolyisocyanates, with the aromatic polyisocyanates being most preferred.Representative examples of the aliphatic polyisocyanates aretrimethylene, tetramethylene, tetramethylxylylene, pentamethylene,hexamethylene, 1,2-propylene, 1,2-butylene, 2,3-butylene, and1,3-butylene diisocyanates. Also suitable are the cycloalkylenecompounds such as 1,3-cyclopentane, 1,4-cyclohexane, 1,2-cyclohexane andisophorone diisocyanates. Representative examples of the aromaticpolyisocyanates are m-phenylene, p-phenylene, 4,4-diphenyl,1,5-naphthalene and 1,4-naphthalene diisocyanates anddiphenylmethane-4,4-diisocyanate (MDI), polymericdiphenylmethane-4,4-diisocyanate (crude MDI); the aliphatic-aromaticcompounds such as 2,4- or 2,6-tolylene diisocyanates, or mixturesthereof, 4,4-toluidine, and 1,4-xylylene diisocyanates; thenuclear-substituted aromatic compounds such as dianisidine diisocyanate,4,4-diphenylether diisocyanate and chlorodiphenylene diisocyanate; thetriisocyanates such as triphenylmethane-4,4,4-triisocyanate,1,3,5-triisocyanatobenzene and 2,4,6-triisocyanatotoluene; thetetraisocyanates such as4,4-dimethyldiphenylmethane-2,2,5,5-tetraisocyanate; the polymerizedpolyisocyanates such as tolylene diisocyanate dimers and trimers, andthe like. The preferred isocyanate is toluene diisocyanate.

In addition, the isocyanates can be prepolymers derived front polyolsincluding polyether polyol or polyester polyol, including polyols whichare reacted with excess polyisocyanates to form isocyanate-terminatedprepolymers. These may be simple polyols such as glycols, for example,ethylene glycol and propylene glycol, as well as other polyols such asglycerol, trimethylolpropane, hexanetriol, pentaerythritol, and thelike, as well as ether-alcohols such as diethylene glycol, tripropyleneglycol and the like and polyethers, that is, alkylene oxide condensatesof the above. Among the alkylene oxides that may be condensed with thesepolyols to form polyethers are ethylene oxide, propylene oxide, butyleneoxide, styrene oxide and the like. These are generally calledhydroxy-terminated polyethers and can be linear or branched. Examples ofpolyethers include polyoxyethylene glycol having a molecular weight ofapproximately 1540, polyoxypropylene glycol having a molecular weight ofapproximately 1025, polyoxytetramethylene glycol, polyoxyhexamethyleneglycol, polyoxynonamethylene glycol, polyoxydecamethylene glycol,polyoxydodecamethylene glycol and mixtures thereof. Other types ofpolyoxyalkylene glycol ethers can be used. Especially useful polyetherpolyols are those derived from reacting polyols such as ethylene glycol,diethylene glycol, triethylene glycol, 1,4-butylene glycol, 1,3-butyleneglycol, 1,6-hexanediol, and their mixtures; glycerol, trimethylolethane,trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol,dipentaerythritol, tripentaerythritol, polypentaerythritol, sorbitol,methyl glucosides, sucrose and the like with alkylene oxides such asethylene oxide, propylene oxide, their mixtures, and the like.

The blocking agent of the blocked tsocyanate groups can be an alcohol,including glycol monoethers and amino alcohols. Examples includealiphatic alcohols such as methanol and 2-ethylhexyl alcohol;cycloaliphatic alcohols such as cyclohexanol; aromatic alkyl alcoholssuch as benzyl alcohol; glycol monoethers such as the monoalkyl ethersof ethylene glycol, i.e. the monobutyl ether of diethylene Elycol; andamino alcohols such as dimethylethanolamine. Oximes such as methyl ethylketoxime; lactams such as epsilon-caprolactam; aliphatic amines such asdibutylamine and beta-dicarbonyl compounds such as acetyl acetone canalso be used. Preferably, the blocking agent is the monobutyl ether ofdiethylene glycol.

As mentioned above, Y or Y' is a residue derived from an epoxy resin.The epoxy resin is preferably a polyepoxide having a 1,2-epoxyequivalency of 2 or more, most preferably about 2. The preferredpolyepoxides are polyglycidyl ethers of cyclic polyols. Particularlypreferred are polyglycidyl ethers of polyphenols such as bisphenol A.

Besides the polyglycidyl ethers of polyphenols, epoxy-containing acrylicpolymers can be used. These polymers are formed by polymerizing anunsaturated epoxy group-containing monomer such as glycidyl acrylate orglycidyl methacrylate with one or more polymerizable ethylenicallyunsaturated monomers. Examples of these polymers are described in U.S.Pat. No. 4,001,156.

Several different reaction schemes can be used to prepare the curingagents of the present invention. These are described in the Journal ofCoatings Technology, volume 55 number 700, page 49-57 (1983). Twosynthetic schemes are preferred. These are modifications of the routes 1and 2 cited in the above reference. In one route (synthetic route 2), afully blocked polyisocyanate is reacted with a polyepoxide in thepresence of a catalyst. It is believed that the blocking agent isliberated from the fully blocked isocyanate and an oxazolidone linkagebetween the polyepoxide and the polyisocyanate is formed. The resultingreaction product is a mixture of materials comprising: a fully blockedisocyanate; a reaction product derived from the polyepoxide moietiesterminated with oxazolidone groups which are in turn linked to blockedisocyanate groups; and polymeric species containing repeatingoxazolidone groups and units derived from the polyepoxide. In preparingthe curing agents of the present invention, the molar ratio of blockedisocyanate to epoxide is at least 1.1:1.0, but not more than 10.0:1.0;preferably, the molar ratio is between 1.5-5.0:1.0. The use of theseratios results in products which are terminated predominantly withblocked isocyanate groups.

In a preferred reaction, toluene diisocyanate which has been fullyblocked with the monobutyl ether of diethylene glycol is reacted in thepresence of triethylamine catalyst with the diglycidyl ether ofbisphenol A in a molar ratio of 1.75-2.0:1.0. These materials arereacted under a nitrogen atmosphere at 90°-130° C. until analysisindicates substantially complete reaction of the epoxide groups.

Another preferred route (synthetic route 1 in the above reference) formaking the oxazolidone-containing curing agents of this invention is byreaction of a partially blocked polyisocyanate with a polyepoxide. Inthis case, a polyisocyanate containing unblocked isocyanate groups andblocked isocyanate groups is reacted with a polyepoxide in the presenceof a catalyst. It is believed that the unblocked isocyanate and epoxidereact to form a reaction product containing an oxazolidone linkage andblocked isocyanate groups.

Tile oxazolidone-containing blocked isocyanate curing agents describedabove cause substantially less ED-related yellowing in subsequentlyapplied topcoats than diblocked TDI which has not been reacted withpolyepoxides. Further, the curing agents cause less yellowing than TDIadducts in which the TDI is adducted with trimethylolpropane (3:1 molarratio) and the isocyanate groups blocked with a conventional blockingagent such as the monobutyl ether of diethylene glycol.

The active hydrogen-containing ionic resin is present in the cationicelectrodepositable coating composition in amounts of about 20 to 99.5percent, preferably 30 to 70 percent based on the weight of resin solidspresent in the electrodepositable coating composition. Accordingly, theamount of curing agent is from 0.5 to 80 percent, preferably 70 to 30percent based on the weight of resin solids present in theelectrodepositable coating composition.

Tile electrodepositable coating compositions of the present inventionare in the form of aqueous dispersions. The term "dispersion" refers toa two-phase transparent, translucent or opaque resinous system in whichthe resin is in the dispersed phase and the water is in the continuousphase. The average particle size of the resinous phase is generally lessthan 10 and usually less than 5 microns, preferably less than 0.5microns, The concentration of the resinous phase in the aqueous media isusually at least 1 percent and usually from about 2 to 60 percent byweight based on weight of the aqueous dispersion. When the compositionsof the present invention are in the form of resin concentrates, theygenerally have a resin solids content of about 26 to 60 percent byweight based on the weight of the aqueous dispersion. When thecompositions of the present invention are in the form ofelectrodepositable coating compositions, the resin solids content of thecomposition is usually within the range of about 5 to 25 percent byweight based on weight of the electrodepositable coating composition.

Besides water, the aqueous medium may contain a coalescing solvent.Useful coalescing solvents include hydrocarbons, alcohols, esters,ethers and ketones. The preferred coalescing solvents include alcohols,polyols and ketones. Specific coalescing solvents include isopropanol,butanol, 2-ethylhexanol, isophorone, 4-methoxy-pentanone, ethylene andpropylene glycol and the monoethyl, monobutyl and monohexyl ethers ofethylene glycol. The amount of coalescing solvent is generally betweenabout 0.01 and 25 percent, and when used, preferably from about 0.05 toabout 5 percent by weight based on the weight of the electrodepositablecoating composition.

In some instances, a pigment composition and if desired, variousadditives such as surfactants, wetting agents or catalysts are includedin the dispersion. The pigment composition may be of the conventionaltypes comprising, for example, iron oxides, lead oxides, strontiumchromate, carbon black, coal dust, titanium dioxide, talc, bariumsulfate, as well as color pigments such as cadmien yellow, cadmium red,chromium yellow and the like. The pigment content of the dispersion isusually expressed as a pigment-to--resin ratio. In the practice of thepresent invention, the pigment-to-resin ratio is usually within therange of 0.02:1 to 1:1. The other additives mentioned above are usuallyin the dispersion in amounts of about 0.01 to 3 percent by weight basedon the total solids weight of electrodepositable resins present in theelectrodepositable coating composition.

When the electrodepositable coating composition described above isemployed for use in electrodeposition, the coating composition is placedin contact with an electrically conductive anode and an electricallyconductive cathode with the surface to be coated being the either thecathode or the anode, depending on whether the ionic activehydrogen-containing resin is anionic or cationic; preferably theelectrode to be coated is the cathode. Following contact with thecoating composition, an adherent film is deposited on one electrode whena sufficient voltage is impressed between the electrodes. The conditionsunder which electrodeposition is carried out are, in general, similar tothose used in electrodeposition of other types of coatings. The appliedvoltage may be varied and can be, for example, as low as 1 volt to ashigh as several thousand volts, but typically is between 50 and 500volts. The current density is usually between 0.5 and 5 amperes persquare foot and tends to decrease during electrodeposition indicatingthe formation of an insulating film. The coating compositions of thepresent invention can be applied to a variety of electroconductivesubstrates especially metals such as steel, aluminum, copper, magnesiumand conductive carbon coated materials.

After the coating has been applied by electrodeposition, it is curedusually by baking at elevated temperatures for example at 90°-260° C.for 1 to 40 minutes.

Illustrating the invention are the following examples which, however,are not to be considered as limiting the invention to their details. Allparts and percentages in the examples as well as throughout thespecification are by weight unless otherwise indicated. Parts by weightare in grams unless otherwise indicated.

EXAMPLES

The following examples show the formulation of electrodepositablecoating compositions with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic isocyanate groups andoxazolidone groups. For the purposes of comparison, electrodepositablecoating compositions were prepared with the same activehydrogen-containing cationic resin and curing agents containing blockedaromatic isocyanate groups but without the oxazolidone groups.

Examples A-B show the preparation of a modifying resin and an activehydrogen-containing resin used to make the electrodepositable coatingcompositions. Examples I-VI show the preparation of various curingagents used to illustrate the invention, the preparation ofelectrodepositable compositions and the use of these compositions in theprocess of electrodeposition.

The electrodepositable coating compositions were tested for theirtendency to cause fame-yellowing in subsequently applied topcoat systemsby the following method: the electrodepositable coating compositionswere electrocoated onto open-ended 1/2-pint rimless steel cans. Theelectrocoated cans were sealed with aluminum foil, then the coatings onthe cans were cured. Any volatiles released from the coatings weretrapped within the foil-sealed cans. The foil was then removed from oneend of the electrocoated cans and the cans were placed on steel panelswhich were previously coated with an aminoplast-polyester-acrylicsprayable white topcoat, commercially available from PPG Industries,Inc. as DHT-5920. A spacer, such as a paper clip, was placed under therim of the can to produce a small vent from the inside of the can. Thesteel panels with the cans on them were then baked so as to cure thetopcoat in the areas of the steel panels that are not covered by theelectrocoated cans. The topcoat in the areas of the steel panels coveredby the electrocoated cans were found to be only partially cured. Thepartially cured topcoat in the areas of the steel panels covered by theelectrocoated cans were then evaluated visually for yellowness andranked on a scale of zero to ten where zero represents the most yellowedtopcoat color and ten represents an unyellowed topcoat color.

Table I shows the results of a comparison of the electrodepositablecoating compositions of Examples I-VI for fume yellowing.

EXAMPLE A Preparation of a Modifying Resin

A polyepoxide polyoxyalkylenediamine modifying resin was prepared. Themodifying resin provides better appearance in the cured electrodepositedcoating. In order to prepare the modifying resin, an intermediatepolyepoxide was prepared from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        EPON 828.sup.1      1000.0                                                    Bisphenol A.sup.2   308.1                                                     Ethyltriphenyl phosphonium iodide                                                                 1.3                                                       2-Butoxyethanol     413.5                                                     ______________________________________                                         .sup.1 The diglycidyl ether of bisphenol A which is commercially availabl     from Shell Corporation.                                                       .sup.2 A polyphenol commercially available from Shell Corporation.       

The EPON 828 and the bisphenol A were charged to a reaction vessel undera nitrogen blanket and heated to 110° C. The reaction mixture was heldat 110° C. until all the bisphenol A had dissolved whereupon theethyltriphenyl phosphonium iodide catalyst was added and the reactionmixture was heated to 160° C. where it was held for an hour to completereaction. When the hold was over, the 2-butoxyethanol was added to givea solids content of 76% and an epoxy equivalent of 504 (based onsolids.)

The modifying resin was then prepared from the following mixture ofingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        JEFFAMINE D-2000.sup.1                                                                            2362.2                                                    Polyepoxide intermediate prepared                                                                 1141.6                                                    as described above                                                            2-Butoxyethanol     296.1                                                     88% Aqueous Lactic Acid                                                                           96.6                                                      Deionized Water     5279.1                                                    ______________________________________                                         .sup.1 A polyoxypropylenediamine having a molecular weight of 2000,           commercially available from Texaco Chemical Company.                     

The JEFFAMINE D-2000 was charged to a reaction vessel under a nitrogenatmosphere and heated to 90° C. The polyepoxide intermediate and the2-butoxyethanol were added over a one-half hour period. At thecompletion of the addition, the reaction mixture was heated to 130° C.and held for three hours. The resin was then dispersed by pouring itinto a mixture of 88% lactic acid and deionized water. The resultingreaction product had a solids content of 36%.

EXAMPLE B Preparation of an Active Hydrogen-Containing Resin

An active hydrogen-containing resin was prepared from the followingmixture of ingredients:

    ______________________________________                                        INGREDIENTS      PARTS BY WEIGHT                                              ______________________________________                                        EPON 828         2729.20                                                      Bisphenol A      791.20                                                       MACOL 98.sup.1   971.60                                                       Methyl Isobutyl Ketone                                                                         160.00                                                       Benzyldimethylamine                                                                            4.04                                                         Benzyldimethylamine                                                                            10.78                                                        Methyl Isobutyl Ketone                                                                         985.50                                                       Diketimine.sup.2 292.28                                                       N-methylethanolamine                                                                           257.79                                                       ______________________________________                                         .sup.1 Polyol derived from bisphenol A and ethylene oxide, commercially       available from Mazur Chemical Company at 100% solids.                         .sup.2 Diketimine derived from diethylene triamine and methyl isobutyl        ketone and supplied at 72% solids in methyl isobutyl ketone.             

The EPON 828, bisphenol A, MACOL 98 and methyl isobutyl ketone werecharged into a reaction vessel and heated under a nitrogen atmosphere to140° C. The first portion of the benzyldimethylamine was added and thereaction mixture allowed to exotherm to 155° C. The reaction mixture washeated to 160° C., held until an epoxy equivalent weight of 626.9 wasobtained, cooled further to 145° C. and the second portion ofbenzyldimethylamine added. The reaction was held at 145° C. until anepoxy equivalent weight of 1054.5 was obtained. At this point, 985.5 gof methyl isobutyl ketone was added and the reaction mixture cooled to114° C. and the diketimine and the N-methylethanolamine were added insuccession. The mixture was allowed to exotherm and then a temperatureof 125° C. was established and held for one hour. Finally, the mixturewas cooled and poured out.

EXAMPLE I (COMPARATIVE) Preparation of an Electrodepositable CoatingComposition Using Diblocked TDI

This example illustrates the preparation of an electrodepositablecoating composition with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic isocyanate groups and nooxazolidone groups. The aromatic isocyanate groups are represented bythe diblocked toluene diisocyanate (TDI). This is a comparative examplewhich shows the degree to which electrodepositable coating compositionsof the prior art produced fume yellowing in subsequently appliedtopcoats.

A diblocked isocyanate curing agent based on TDI was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS      PARTS BY WEIGHT                                              ______________________________________                                        MONDUR TD80.sup.1                                                                              435.00                                                       Butyl CARBITOL.sup.2                                                                           810.00                                                       Methyl Isobutyl Ketone                                                                         415.00                                                       ______________________________________                                         .sup.1 An 80/20 mixture of 2,4 and 2,6toluene diisocyanates (TDI)             commercially available from Miles Chemical Company.                           .sup.2 The monobutyl ether of diethylene glycol commercially available        from Union Carbide Corporation.                                          

The butyl CARBITOL and methyl isobutyl ketone were charged to a reactionflask under a nitrogen atmosphere and heated to 85° C. The MONDUR TD80isocyanate was added slowly while holding the temperature at 85°-100° C.until analysis by infrared spectroscopy indicated no unreactedisocyanate remained.

As electrodepositable resin was prepared which contains the curing agentdescribed above from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS       PARTS BY WEIGHT                                             ______________________________________                                        Active Hydrogen-Containing Base Resin                                                           700.00                                                      of Example B                                                                  Curing Agent, as prepared above                                                                 313.24                                                      100% Sulfamic Acid                                                                              39.37                                                       Deionized Water   1543.45                                                     ______________________________________                                    

The active hydrogen-containing base resin an the curing agent werecharged to a reaction flask under a nitrogen atmosphere and heated to80° C. The heat was turned off and the nitrogen was removed, then thesulfamic acid was added followed by the slow addition of the deionizedwater while the temperature decreased to 45° C. The aqueous dispersionwas heated to 50°-55° C. under a vacuum of 22-25 inches of Hg and 233grams of a distillate comprising methyl isobutyl ketone and water wereremoved to produce a dispersed resin with a solids content of 34.96%.

An electrodepositable coating composition was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS        PARTS BY WEIGHT                                            ______________________________________                                        Electrodepositable Resin as                                                                      1493.40                                                    prepared above                                                                Modifying Resin of Example A                                                                     153.70                                                     PARAPLEX WP-1.sup.1                                                                              36.90                                                      Hexyl CELLOSOLVE.sup.2                                                                           10.00                                                      E5994 Paste.sup.3  398.70                                                     Deionized Water    1707.30                                                    ______________________________________                                         .sup.1 A propoxylated cresol, commercially available from Rohm and Haas.      .sup.2 The monohexyl ether of diethylene glycol, commercially available       from Union Carbide Corporation.                                               .sup.3 A milled dispersion commercially available from PPG Industries,        Inc. which contains 46.2% titanium dioxide, 1.5% carbon black, 3.4% basic     lead silicate, and 2.6% dibutyltin oxide; the percentages being by weight     based on the total weight of the pigment paste.                          

This coating composition was electrodeposited onto a 1/2-pint rimlesssteel can, with said can acting as the cathode. The coating compositionwas heated to 85° F. (29° C.) under gentle agitation, then a voltage of200 volts was impressed between the electrodes for two minutes toachieve a film build of about 1.1 mils (0.0028 cm). The rimless steelcan was removed from the coating composition, rinsed with a spray of tapwater, then baked to produce a cured film. The coated cans wereevaluated for fume yellowing and the results are reported in Table Ibelow.

EXAMPLE II (COMPARATIVE) Preparation of an Electrodepositable CoatingComposition Using Modified TDI

This example illustrates the preparation of an electrodepositablecoating composition with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic tsocyanate groups and nooxazolidone groups. The aromatic isocyanate groups are contributed byTDI, which has been modified by adduct formation withtrimethylolpropane. The unreacted isocyanate groups are blocked with themonobutyl ether of diethylene glycol. This is a comparative examplewhich shows the degree to which electrodepositable coating compositionsof the prior art which contain higher molecular weight curing agentsproduce fume yellowing in subsequently applied topcoats.

A fully blocked polyisocyanate curing agent based on TDI was preparedfrom the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS      PARTS BY WEIGHT                                              ______________________________________                                        MONDUR TD80      1044.00                                                      Methyl Isobutyl Ketone                                                                         381.00                                                       Butyl CARBITOL   973.33                                                       Methyl Isobutyl Ketone                                                                         381.00                                                       Dibutyltin Dilaurate                                                                           0.58                                                         Trimethylolpropane                                                                             89.40                                                        Trimethylolpropane                                                                             89340                                                        Trimethylolpropane                                                                             89.40                                                        Dibutyltin Dilaurate                                                                           0.18                                                         ______________________________________                                    

The MONDUR TD80 and 381.00 g of methyl isobutyl ketone were charged to areaction flask. The butyl CARBITOL was added over a three hour periodwhile maintaining a temperature of no more than 60° C. Upon completionof the addition, the reaction mixture was held at 65° C. for 60 minutes.The dibutyltin dilaurate and 381.00 grams of methyl isobutyl ketone wereadded while the mixture was held at 80° C. The material was then cooledto 74° C. and held for 20 minutes. The first portion of thetrimethylolpropane was added while maintaining the temperature at 75°C., then the material was held at this temperature for 20 minutes. Thesecond portion of the trimethylolpropane was added while maintaining thetemperature at 85° C., then the material was held at this temperaturefor 20 minutes. The final portion of the trimethylolpropane was addedwhile maintaining the temperature at 96° C., then the material was heldat this temperature for 20 minutes. The material was then heated to 105°C. and held for one hour. The 0.18 g of dibutyltin dilaurate were addedand the material was held at 105° C. until analysis by infraredspectroscopy indicated no unreacted isocyanate remained.

An electrodepositable resin was prepared which contained the curingagent described above from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        Active Hydrogen-Containing Resin                                                                  700.0                                                     of Example B                                                                  Curing Agent, as prepared above                                                                   478.91                                                    Sulfamic Acid       39.37                                                     Deionized Water     1754.31                                                   ______________________________________                                    

The active hydrogen-containing resin and the curing agent were chargedto a reaction flask under a nitrogen atmosphere and heated to 80° C. Theheat was turned off and the nitrogen was removed, then the sulfamic acidwas added followed by slow addition of deionized water while thetemperature decreased to 45° C. The aqueous dispersion was heated to50°-55° C. under a vacuum of 22-25 inches of Hg and 278 grams of adistillate comprising methyl Isobutyl ketone and water were removed toproduce a dispersed resin with a solids content of 37.24%.

An electrodepositable coating composition was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        Electrodepositable Resin as prepared                                                               1405.10                                                  above                                                                         Modifying Resin of Example A                                                                       153.70                                                   PARAPLEX WP-1        36.90                                                    Hexyl CELLOSOLVE     10.00                                                    E5994 Paste          398.70                                                   Deionized Water      1795.60                                                  ______________________________________                                    

This coating composition was electrodeposited onto a 1/2-pint rimlesssteel can, cured and evaluated for fume yellowing as generally describedin Example I.

EXAMPLE III (COMPARATIVE) Preparation of an Electrodepositable CoatingComposition Using Modified Crude MDI

This example illustrates the preparation of an electrodepositablecoating composition with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic isocyanate groups and nooxazolidone groups. The aromatic isocyanate groups are derived fromfully-blocked crude MDI, which has a higher molecular weight than TDI.This is a comparative example which shows the degree to whichelectrodepositable coating compositions of the prior art which containhigher molecular weight curing agents produce fame yellowing insubsequently applied topcoats.

A fully blocked polyisocyanate curing agent based on crude MDI wasprepared from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS      PARTS BY WEIGHT                                              ______________________________________                                        MONDUR MRS-4.sup.1                                                                             4095.0                                                       Methyl Isobutyl Ketone                                                                         2199.6                                                       Dibutyltin dilaurate                                                                           6.2                                                          2-(2-Butoxyethoxy)ethanol                                                                      3353.0                                                       Trimethylol propane                                                                            356.1                                                        2-(2-Butoxyethoxy)ethanol                                                                      464.2                                                        ______________________________________                                         .sup.1 Polymeric diphenylmethane diisocyanate crude MDI commercially          available Mobay Chemical Company.                                        

The MONDUR MRS-4, methyl isobutyl ketone and dibutyltin dilaurate werecharged to a reaction flask and heated under a nitrogen atmosphere to30° C. The first portion of the 2-(2-butoxyethoxy)ethanol was addedslowly while holding the temperature between 60° and 65° C. Uponcompletion of tile addition, the reaction mixture was held at 65° C. for90 minutes, The trimethylol propane was then added and the mixtureheated to 110° C. and held for three hours whereupon the final portionof the 2-(2-butoxyethoxy)ethanol was added. The 110° C. hold wascontinued until analysis by infrared spectroscopy indicated no unreactedisocyanate remained.

An electrodepositable resin was prepared which contains the curing agentdescribed above from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        Active Hydrogen-Containing Resin                                                                  700.00                                                    of Example B                                                                  Curing Agent, as prepared above                                                                   410.36                                                    Sulfamic Acid       39.37                                                     Deionized Water     1716.80                                                   ______________________________________                                    

The active hydrogen-containing resin and the curing agent were chargedto a reaction flask under a nitrogen atmosphere and heated to 80° C. Theheat was turned off and the nitrogen was removed, then the sulfamic acidwas added followed by slow addition of deionized water while thetemperature decreased to 45° C. The aqueous dispersion was heated to50°-55° C. under a vacuum of 22-25 inches of Hg and 298 grins of adistillate comprising methyl isobutyl ketone and water were removed toproduce a dispersed resin with a solids content of 36.21%.

An electrodepositable coating composition was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        Electrodepositable Resin as prepared                                                               1443.90                                                  above                                                                         Modifying Resin of Example A                                                                       153.70                                                   PARAPLEX WP-1        36.90                                                    Hexyl CELLOSOLVE     10.00                                                    E5994 Paste          398.70                                                   Deionized Water      1756.80                                                  ______________________________________                                    

This coating composition was electrodeposited onto a 1/2-pint rimlesssteel can, cured and evaluated for fume yellowing as generally describedin Example I.

EXAMPLE IV Preparation of an Electrodepositable Coating CompositionUsing an Oxazolidone Group-Containing Curing Agent

This example illustrates the preparation of an electrodepositablecoating composition with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic isocyanate groups andoxazolidone groups. The aromatic isocyanate groups are contributed byTDI which is blocked with the monobutyl ether of diethylene glycol. Thisexample shows the degree to which the electrodepositable coatingcompositions of the preferred embodiment of the present invention resistED-related yellowing in subsequently applied topcoats.

An isocyanate curing agent based on TDI and also containing oxazolidonegroups was prepared. First, partially blocked TDI was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS    PARTS BY WEIGHT                                                ______________________________________                                        Butyl CARBITOL 1944.00                                                        MONDUR TD80    2088.0                                                         Dibutyltin Dilaurate                                                                         2 drops                                                        ______________________________________                                    

The MONDUR TD80 was charged to a reaction flask under a nitrogenatmosphere. The butyl CARBITOL was added slowly and the temperature wasmaintained at 35°-40° C. After four hours, the two drops of dibutyltindilaurate was added. No exotherm was observed. The temperature wasmaintained at 30° C. for another hour, at which time analysis showedthat 60.2% of the isocyanate groups, which were initially present,remained. The sample was held at room temperature overnight and analysisshowed 53.2% of the initially present isocyanate groups remained.

An oxazolidone group-containing curing agent was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        EPON 828             376.00                                                   TDI, partially blocked with                                                                        694.30                                                   Butyl CARBITOL, as prepared above                                             Tetraethylammonium Bromide                                                                         1.20                                                     Methyl Isobutyl Ketone                                                                             357.20                                                   ______________________________________                                    

The EPON 828 and the tetraethylammonium bromide were charged to areaction flask under a nitrogen atmosphere and heated to 160° C. Thepartially blocked TDI was added slowly while holding the temperature at160° C. After the addition, the reaction mixture was held at 160° C.until analysis by infrared spectroscopy indicated no unreactedtsocyanate remained and until an epoxy equivalent weight of 12,372.1 wasreached. The methyl isobutyl ketone was added while the mixture wascooling.

An electrodepositable resin was prepared which contains the curing agentdescribed above from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        Active Hydrogen-Containing Resin                                                                  700.00                                                    of Example B                                                                  Curing Agent, as prepared above                                                                   612.64                                                    Sulfamic Acid       39.37                                                     Deionized Water     1924.51                                                   ______________________________________                                    

The active hydrogens-containing resin and the curing agent were chargedto a reaction flask under a nitrogen atmosphere and heated to 80° C. Theheat was turned off and the nitrogen was removed, then the sulfamic acidwas added followed by slow addition of deionized water while thetemperature decreased to 45° C. The aqueous dispersion was heated to50°-55° C. under a vacuum of 22-25 inches of Hg and 353 grams of adistillate comprising methyl isobutyl ketone and water were removed toproduce a dispersed resin with a solids content of 36.49%.

An electrodepositable coating composition was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        Electrodepositble Resin as prepared                                                                1432.10                                                  above                                                                         Modifying Resin of Example A                                                                       153.70                                                   PARAPLEX WP-1        36.90                                                    Hexyl CELLOSOLVE     10.00                                                    E5994 Paste          398.70                                                   Deionized Water      1768.60                                                  ______________________________________                                    

This coating composition was electrodeposited onto a 1/2-pint rimlesssteel can, cured and evaluated for fume yellowing as generally describedin Example I.

EXAMPLE V Preparation of an Electrodepositable Coating Composition Usingan Oxazolidone Curing Agent with an Alternate Blocking Agent

This example illustrates the preparation of an electrodepositablecoating composition with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic isocyanate groups andoxazolidone groups. The aromatic tsocyanate groups are derived from TDIwhich is blocked with 2-butoxyethanol. This example shows the degree towhich the electrodepositable coating compositions of the presentinvention resist ED-related yellowing in subsequently applied topcoats.

An isocyanate curing agent based on TDI and also containing oxazolidonegroups was prepared. This curing agent was blocked with 2-butoxyethanol.First, partially blocked TDI was prepared from the following mixture ofingredients:

    ______________________________________                                        INGREDIENTS      PARTS BY WEIGHT                                              ______________________________________                                        Butyl CELLOSOLVE.sup.1                                                                         590.00                                                       MONDUR TD80      870.00                                                       ______________________________________                                         .sup.1 2butoxyethanol, commercially available from Union Carbide Corp.   

The MONDUR TD80 was charged to a reaction flask under a nitrogenatmosphere. The butyl CELLOSOLVE was added slowly and the temperaturewas maintained at 35°-40° C. during the addition. After five hours,analysis showed that 56.2% of the isocyanate groups, which wereinitially present, remained. The sample was stored overnight andanalysis showed that 53.7% of the initially present isocyanate groupsremained.

A curing agent was prepared from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        EPON 828            376.00                                                    TDI, partially blocked with                                                                       598.40                                                    butyl CELLUSOLVE, as prepared                                                 above                                                                         Tetraethylammonium Bromide                                                                        1.20                                                      Methyl isobutyl ketone                                                                            325.20                                                    ______________________________________                                    

The EPON 828 and the tetraethylammonium bromide were charged to areaction flask under a nitrogen atmosphere and heated to 160° C. Thepartially blocked TDI was added slowly while holding the temperature at160° C. until analysis by infrared spectroscopy indicated no unreactedisocyanate remained and until an epoxy equivalent weight of 20,000+ wasreached. The methyl isobutyl ketone was added while the mixture wascooling.

An electrodepositable resin was prepared which contains the curing agentdescribed above from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        Active Hydrogen-Containing Resin                                                                  700.00                                                    of Example B                                                                  Curing Agent, as prepared above                                                                   557.83                                                    Sulfamic Acid       39.37                                                     Deionized Water     1854.75                                                   ______________________________________                                    

The active hydrogen-containing resin and the curing agent were chargedto a reaction flask under a nitrogen atmosphere and heated to 80° C. Theheat was turned off and the nitrogen was removed, then the sulfamic acidwas added followed by the slow addition of deionized water while thetemperature decreased to 45° C. The aqueous dispersion was heated to50°-55° C. under a vacuum of 22-25 inches of Hg and 378 grams of adistillate comprising methyl isobutyl ketone and water were removed toproduce a dispersed resin with a solids content of 36.29%.

An electrodepositable coating composition was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        Electrodepositable Resin as prepared                                                               1439.90                                                  above                                                                         Modifying Resin of Example A                                                                       153.70                                                   PARAPLEX WP-1        36.90                                                    Hexyl CELLOSOLVE     10.00                                                    E5994 Paste          398.70                                                   Deionized Water      1760.80                                                  ______________________________________                                    

This coating composition was electrodeposited onto a 1/2-pint rimlesssteel can, cured and evaluated for fume yellowing as generally describedin Example I.

EXAMPLE VI Preparation of an Electrodepositable Coating CompositionUsing an Oxazolidone Curing Agent Prepared With The Diglycidyl Ether ofPolypropylene Glycol

This example illustrates the preparation of an electrodepositablecoating composition with an active hydrogen-containing cationic resinand a curing agent containing blocked aromatic isocyanate groups andoxazolidone groups. The aromatic isocyanate groups are contributed byTDI which is blocked with the monobutyl ether of diethylene glycol. Thecuring agent used to make this resin was prepared with the diglycidylether of polypropylene glycol. This example shows the degree to whichthe electrodepositable coating compositions of the present inventionresist ED-related yellowing tn subsequently applied topcoats.

An isocyanate curing agent based on TDI and also containing oxazolidonegroups was prepared. This curing agent was prepared with the diglycidylether of polypropylene glycol. First, partially blocked TDI was preparedfrom the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS    PARTS BY WEIGHT                                                ______________________________________                                        Butyl CARBITOL 1944.00                                                        MONDUR TD80    2088.0                                                         Dibutyltin Dilaurate                                                                         2 drops                                                        ______________________________________                                    

The MONDUR TD80 was charged to a reaction flask under a nitrogenatmosphere. The butyl CARBITOL was added slowly anti the temperature wasmaintained at 35°-40° C. After four hours, the two drops of dibutyltindilaurate was added. No exotherm was observed. The temperature wasmaintained at room temperature overnight after which time the NCOequivalent weight was 315.6.

A curing agent was prepared from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        DER736.sup.1         338.20                                                   TDI, partially blocked with                                                                        694.30                                                   Butyl CARBITOL, as prepared above                                             Tetraethylammonium Bromide                                                                         1.20                                                     Methyl isobutyl ketone                                                                             344.60                                                   ______________________________________                                         .sup.1 Diglycidyl ether of polypropylene glycol, commercially available       from Dow Chemical Co. at 100% solids.                                    

The DER736 and the tetraethylammonium bromide were charged to a reactionflask under a nitrogen atmosphere and heated to 160° C. The partiallyblocked TDI was added slowly while holding the temperature at 160° C.until analysis by infrared spectroscopy indicated no unreactedisocyanate remained and until an epoxy equivalent weight of 20,000+ wasreached. The methyl isobutyl ketone was added while the mixture wascooling.

An electrodepositable resin was prepared which contains the curing agentdescribed above from the following mixture of ingredients:

    ______________________________________                                        INGREDIENTS         PARTS BY WEIGHT                                           ______________________________________                                        Active Hydrogen-Containing Resin                                                                  700.00                                                    of Example B                                                                  Curing Agent, as prepared above                                                                   591.13                                                    Sulfamic Acid       39.37                                                     Deionized Water     1897.13                                                   ______________________________________                                    

The active hydrogen-containing resin and the curing agent were chargedto a reaction flask under a nitrogen atmosphere and heated to 80° C. Theheat was turned off and the nitrogen was removed, then the sulfamic acidwas added followed by the slow addition of the deionized water while thetemperature decreased to 45° C. The aqueous dispersion was heated to50°-55° C. under a vacuum of 22-25 inches of Hg and 415 grams of adistillate comprising methyl isobutyl ketone and water were removed toproduce a dispersed resin with a solids content of 37.46%.

An electrodepositable coating composition was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        INGREDIENTS          PARTS BY WEIGHT                                          ______________________________________                                        Electrodepositable Resin as prepared                                                               1393.80                                                  above                                                                         Modifying Resin of Example A                                                                       153.70                                                   PARAPLEX WP-1        36.90                                                    Hexyl CELLOSOLVE     10.00                                                    E5994 Paste          398.70                                                   Deionized Water      1806.80                                                  ______________________________________                                    

This coating composition was electrodeposited onto a 1/2-pint rimlesssteel can, cured and evaluated for fume yellowing as generally describedin Example I.

                                      TABLE I                                     __________________________________________________________________________    A COMPARISON OF ED-RELATED YELLOWING                                          OF THE COMPOSITIONS OF EXAMPLE I-IV                                                     ELECTROCOAT                                                                              ELECTROCOAT                                                                              ELECTROCOAT                                                                              ELECTROCOAT                                  BAKE SCHEDULE:                                                                           BAKE SCHEDULE:                                                                           BAKE SCHEDULE:                                                                           BAKE SCHEDULE:                               30 MINUTES/                                                                              30 MINUTES/                                                                              30 MINUTES/                                                                              30 MINUTES/                                  340° F. (171° C.)                                                          360° F. (182° C.)                                                          380° F. (193° C.)                                                          400° F. (204°        __________________________________________________________________________                                               C.)                                EXAMPLE I 5.00       3.75       3.00       4.00                               (COMPARATIVE)                                                                 EXAMPLE II                                                                              4.50       3.50       0.00       0.00                               (COMPARATIVE)                                                                 EXAMPLE III                                                                             9.00       8.25       7.75       9.50                               (COMPARATIVE)                                                                 EXAMPLE IV                                                                              6.00       4.50       4.00       8.75                               EXAMPLE V 5.00       4.50       4.00       8.00                               EXAMPLE VI                                                                              9.00       9.00       10.00      10.00                              __________________________________________________________________________     NOTE:                                                                         Yellowness was visually rated on a scale of zero to ten, with zero            representing the most yellowed topcoat and ten representing an unyellowed     topcoat.                                                                 

I claim:
 1. A method for coating a conductive substrate serving as afirst electrode in an electrical circuit, wherein the electrical circuitcomprises said first electrode and a second electrode which are immersedin an aqueous ionic electrodepositable coating composition, said processcomprising passing an electric current between said first and secondelectrodes so as to cause the electrodepositable coating composition todeposit as a coating on said first electrode, wherein saidelectrodepositable coating composition comprises:A. an activehydrogen-containing ionic resin; and B. a curing agent which containsblocked isocyanate groups and groups of the following structure:##STR6## where Y and Y' are different and are hydrogen and a residuederived from an epoxy resin.
 2. The method of claim 1 wherein the firstelectrode is a cathode, the second electrode is an anode and the aqueousionic electrodepositable coating composition is cationic.
 3. The methodof claim 1 wherein the coated substrate is heated to a temperature of90° C. to 260° C. for 1 o 40 minutes to cure the coating.
 4. The methodof claim 1 wherein the active hydrogen containing ionic resin is chosenfrom the group consisting of: acrylic polymers, polyester polymers,polyurethane polymers, polymers derived from epoxy-containing materials,and mixtures thereof.
 5. The method of claim 1 wherein the activehydrogen containing ionic resin is the cationic reaction product of anepoxy group-containing material and an amine.
 6. The method of claim 1wherein the curing agent contains groups of the following structure:##STR7## where X is a residue of an isocyanate and Y and Y' aredifferent and are hydrogen and a residue derived from an epoxy resin. 7.The method of claim 6 wherein X is the residue of an aromaticisocyanate.
 8. The method of claim 6 wherein X is the residue of anaromatic polyisocyanate.
 9. The method of claim 6 wherein the curingagent is of the following structure: ##STR8## where X is a residuederived from a polyisocyanate; and where A is a blocked isocyanate groupof the structure: ##STR9## where D is a radical derived from the groupconsisting of alcohols, lactams, ketoximes, aliphatic amines andbeta-dicarbonyl compounds and where Y and Y' are different and arehydrogen and a residue derived from an epoxy resin.
 10. The method ofclaim 9 in which the epoxy resin is a polyepoxide.
 11. The method ofclaim 9 wherein the polyepoxide is a polyglycidyl ether of a polyhydricphenol.
 12. The method of claim 9 where X is a residue derived from anaromatic polyisocyanate.